1. Field of the Invention
The present invention relates to a radiation imaging system having a function of performing exposure control of a radiation image, and an operating method thereof.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using radiation such as X-rays is known. The X-ray imaging system is constituted of an X-ray generating device for generating X-rays and an X-ray imaging apparatus for acquiring an X-ray image of an object (i.e. patient) from the X-rays that have passed through the object.
The X-ray generating device includes an X-ray source for irradiating the X-rays to the object, a source controller for controlling the operation of the X-ray source, and an irradiation switch for inputting a command for actuating the X-ray source to the source controller. The X-ray source has an X-ray tube, in which thermal electrons emitted from a cathode are made to collide with an anode (target), such that the X-rays are irradiated from a focus located on the anode with which thermal electrons collide. The X-rays are radially irradiated as X-ray beams from the focus to the object. Further, the X-ray generating device includes a displacement mechanism which enables linear movement and rotation of the X-ray source, and is configured to arbitrarily change the X-ray irradiation point and the X-ray irradiation angle of the X-ray source.
The X-ray imaging apparatus includes an X-ray image detecting device for detecting the X-ray image upon receiving the X-rays having passed through the object, and a console for controlling the operation of the X-ray image detecting device and storing and displaying the X-ray image. An X-ray image detecting device using an image detector referred to as a FPD (flat panel detector) has been widely spread. The FPD has an imaging surface, on which pixels for accumulating electric charges corresponding to the received X-ray dose are arranged in a matrix, and detects the X-ray image as the electrical signal. The image detector accumulates signal charges on a pixel-by-pixel basis, and converts the accumulated electric charges into a voltage signal by a signal processing circuit. Thereby, the image detector detects the X-ray image of the object, and outputs the X-ray image as digital image data.
There are a stationary-type X-ray image detecting device and a portable-type X-ray image detecting device. In the stationary-type X-ray image detecting device, the image detector is incorporated into an imaging table. In the portable-type X-ray image detecting device, the image detector is incorporated into a potable flat housing. The stationary-type X-ray image detecting device is intended for imaging a patient in a standing posture, or intended for imaging a patient in a lying posture. In the case where the stationary-type X-ray image detecting device is intended for imaging a patient in a standing posture, the image detector is movable in a vertical direction. In the case where the stationary-type X-ray image detecting device is intended for a patient in a lying posture, the image detector is movable in a horizontal direction. In accordance with a body frame of the patient and a body portion to be imaged, the position of the image detector is adjusted.
The portable-type X-ray image detecting device is referred to as an electronic cassette. The electronic cassette is detachably attached to an existing imaging stand designed for a film cassette and an imaging plate (IP) cassette, or a dedicated imaging table, and used. Otherwise, the electronic cassette may be used independently without using the imaging table. For example, the electronic cassette may be brought into a hospital room of a patient who cannot move to an X-ray examination room, and put on a bed on which the patient is lying, so as to be used. In order to perform X-ray imaging in the hospital room, a medical equipment carrier as the X-ray generating device mounted on a wagon so as to be movable, and the electronic cassette are brought into the hospital room. The electronic cassette is inserted between the patient's body and the bed, and the medical equipment carrier is placed beside the bed.
Irrespective of whether the X-ray image detecting device is the stationary-type or the portable-type, positioning operation for determining a relative positional relationship among the X-ray source, the X-ray image detecting device, and the body part of the patient to be imaged is performed as preparation for the X-ray imaging, prior to the X-ray imaging. For example, at first, the relative position between the patient and the X-ray image detecting device is adjusted such that the center of the body part of the patient to be imaged and center of the imaging surface of the image detector are coincident with each other. Next, the relative position between the X-ray source and the X-ray image detecting device is adjusted such that the center of the imaging surface of the image detector and the center of the X-ray beam (hereinafter referred to as main ray) irradiating from the X-ray source are coincident with each other. The main ray corresponds to the center of the irradiation range of the X-ray source. Accordingly, the relative positional relationship among the X-ray source, the imaging surface of the image detector, and the body part of the patient to be imaged is determined appropriately.
Further, some X-ray imaging systems perform an automatic exposure control (hereinafter, referred to as AEC), in which the dose of X-rays having passed through the patient is detected by a dose detection sensor, and when an integrated value of the dose of X-rays achieves a predetermined threshold level, irradiation of the X-ray by the X-ray generating device is stopped, in order to perform exposure control of the X-ray image (see, for example, Japanese Patent Application Laid-open No. 2011-139761 and U.S. Pat. No. 5,539,798 (corresponding to Japanese Patent Application Laid-open No. 6(1994)-217973). Japanese Patent Application Laid-open No. 2011-139761 discloses an X-ray imaging system including an X-ray image detecting device using an image detector, and the X-ray imaging system performs the AEC using some of pixels in an imaging surface of the image detector as dose detection sensors. In the AEC, at least one of the dose detection sensors is selected, and thereby a dose measurement field is set as a field in which the dose of X-ray is measured. In the AEC, since it is necessary to measure the dose of the X-ray having passed through the patient, it is required to set the dose measurement field to be opposed to the body part of the patient to be imaged in the imaging surface of the image detector.
The X-ray imaging system disclosed in Japanese Patent Application Laid-open No. 2011-139761 includes an optical camera provided to the X-ray source, an image processor for extracting a contour of the body part of the patient to be imaged from an optical image of the patient captured by the optical camera, and a dose measurement field setting circuit for setting a dose measurement field within the extracted contour of the body part to be imaged. According to the X-ray imaging system disclosed in Japanese Patent Application Laid-open No. 2011-139761, positioning operation is performed appropriately such that the main ray of the X-ray source, the center of the imaging surface of the image detector, and the center of the body part of the patient to be imaged are coincident with each other, and in such a state, an optical image of the patient is captured by the optical camera, so as to set the dose measurement field within the contour of the body part to be imaged. The body part to be imaged includes many parts such as cephalic region, breast region, and extremities, and therefore the contour varies depending on the body part to be imaged. In the case where the contour of the body part to be imaged is extracted by using the optical camera in the same manner as the X-ray imaging system disclosed in Japanese Patent Application Laid-open No. 2011-139761, it is possible to contain the dose measurement field within the contour of the body part to be imaged even if the contour of the body part to be imaged varies.
U.S. Pat. No. 5,539,798 discloses an X-ray imaging system with use of an X-ray film. According to U.S. Pat. No. 5,539,798, it is also disclosed that an optical image of a patient is captured by using an optical camera provided to an X-ray source, a contour of a body part to be imaged is extracted from the captured optical image through an image processing, and a dose measurement field for AEC is set within the contour of the body part to be imaged in accordance with the extracted contour, as in the case of Japanese Patent Application Laid-open No. 2011-139761.
However, the X-ray imaging system disclosed in each of Japanese Patent Application Laid-open No. 2011-139761 and U.S. Pat. No. 5,539,798 relates to a technique based on the presumption that the X-ray source, the image detector, and the patient are appropriately positioned. In the case where the center of the imaging surface of the image detector and the center of the body part of the patient to be imaged are not coincident with each other, and the image detector and the body part of the patient to be imaged are not appropriately positioned, for example, there is a problem in that the imaging system in each of Japanese Patent Application Laid-open No. 2011-139761 and U.S. Pat. No. 5,539,798 cannot be adopted.
In the case where the image detector and the body part of the patient to be imaged are appropriately positioned, the center of the X-ray source, the center of the image detector, and the center of the body part of the patient to be imaged are coincident with each other. Therefore, the center of the optical image captured by using the optical camera provided to the X-ray source corresponds to the center of the imaging surface of the image detector, and it becomes possible to appropriately set the position of the dose measurement field in the imaging surface of the image detector in accordance with the position of the body part to be imaged in the optical image.
In contrast, in the case where the center of the imaging surface of the image detector and the center of the body part of the patient to be imaged are not coincident with each other, the position of the X-ray source is moved in accordance with the position of the body part of the patient to be imaged such that the X-rays are irradiated to the body part of the patient to be imaged, and therefore the main ray of the X-ray source and the center of the imaging surface of the image detector are not coincident with each other. In this case, the center of the optical image captured by using the optical camera provided to the X-ray source corresponds to the center of the body part of the patient to be imaged, but does not correspond to the center of the imaging surface of the image detector. Accordingly, even if the position of the body part of the patient to be imaged is specified in the optical image, since the position of the body part to be imaged in the optical image does not correspond to the position of the imaging surface of the image detector, it is not possible to set the dose measurement field in the imaging surface appropriately.
Such a problem also occurs in the stationary-type X-ray image detecting device, but the frequency of occurrence of such a problem becomes prominent in the portable electronic cassette. In the case where the electronic cassette is used on the bed as described above, the electronic cassette is inserted between the patient and the bed. However, in some cases, it is not possible to change the posture of the patient to a large extent depending on the condition of the patient. In this case, it is not possible to perform positioning between the patient and the electronic cassette appropriately, and therefore the center of the body part of the patient to be imaged and the center of the imaging surface of the image detector are not coincident with each other.
According to Japanese Patent Application Laid-open No. 2011-139761 and U.S. Pat. No. 5,539,798, the method of setting the dose measurement field in the case where the body part of the patient to be imaged and the image detector are not appropriately positioned is not clearly disclosed nor suggested. Of course, it is possible to set the position of the dose measurement field appropriately in accordance with the position of the body part of the patient to be imaged, by applying the technique of setting the dose measurement field in part of the imaging surface as disclosed in Japanese Patent Application Laid-open No. 2011-139761, such that the dose measurement field can be set to an arbitrary position in the imaging surface by a manual operation. However, there is a problem in that such a manual operation is cumbersome.